Since the time of the discovery of polyhalite ores and to the present known development efforts have been concentrated primarily on hydrothermic methods of processing polyhalite with the production of such fertilizers as potassium sulphate, sulphate of potash magnesia, syngenite. However, these prior art hydrothermic methods are based upon calcining polyhalite wherefrom sodium chloride has been washed out and followed by leaching a resultant bake with water and converting a resultant solution with potassium chloride. Other methods which comprise converting polyhalite wherefrom sodium chloride has not been washed out with potassium chloride at a temperature ranging from 800.degree. to 850.degree. C., ensure a low degree of recovery of useful components into the finished product, are complicated, power-consuming, require expensive facilities and presuppose the presence of alkaline effluents aggravating environmental pollution. For these reasons the aforementioned methods are not applicable commercially.
Recently there has been proposed a number of more advanced methods of processing polyhalite which afford the production of complex phosphatic-potassic (PP), nitrogen-phosphatic-potassic (NPK), nitrogen-phosphatic-potassic-magnesic (NPKMg) fertilizers. However, these prior art methods based upon decomposing polyhalite mixed with phosphate rock with the use of nitric acid and contemplating the production of molten or condensed potassium phosphates therefrom are technologically complicated, difficult to accomplish in terms of equipment design and do not permit recovery of ballast-free fertilizers.
Potassium phosphate is known to be a valuable fertilizer since it contains more than 70% of nutritive substances. Despite this fact, heretofore there has been developed no economically advantageous methods of producing it since potassium hydroxide and potassium carbonate utilized for the production of potassium phosphate are fairly costly products.
When employing a cheaper raw material, such as potassium chloride, a number of problems present themselves which are concerned with the utilization of the resulting secondary chlorine-containing products.
In the present state of the art there are known various methods of producing a complex mineral fertilizer by means of decomposing polyhalite in the presence of a mineral acid. According to one of such methods polyhalite is decomposed in the presence of sulfuric acid followed by separating gypsum and processing a solution comprised of potassium and magnesium sulphates and of sulfuric acid with sodium carbonate or potassium carbonate, subsequently separating the resultant magnesium carbonate, processing a mother liquor, i.e. the liquor obtained after the separation of magnesium carbonate, with ammonium carbonate or a mixture of ammonia and carbon dioxide, separating sodium bicarbonate and subsequently processing the liquor with phosphoric acid or hydrogen phosphate to produce a complex mineral fertilizer comprised of potassium and ammonium sulphates, ammonium phosphate, etc. The solution obtained after separating gypsum is processed with ammonium carbonate or a mixture of ammonia and carbon dioxide, separating the resultant magnesium carbonate and processing the mother liquor with phosphoric acid or hydrogen phosphate to produce a finished product.
A disadvantage of the prior art method described above consists in the multiplicity of stages and operating complexity. Furthermore, the content of ballast ions in the complex mineral fertilizer provided by this method is objectionably high (up to 50%).
Ballast components are chloride-ions and sulphate-ions which are not assimilated by the plants.
The most effective fertilizers are ballast-free, i.e. the type of fertilizers containing only substances (nitrogen, potassium oxide, phosphoric anhydride, magnesium oxide) fully assimilable by the plants.
In recent years expanded the production of ballast-free concentrated complex fertilizers comprised of two or three components has been a markedly general trend. However, current commercial methods for production of potassium nitrate and potassium phosphate are based upon the employment of potassium chloride and involve corrosion of the equipment, apart from generating a need for the utilization of the by-products containing chloride-ion or chlorine. It is therefore of particular interest for the production of ballast-free potassium fertilizers to make use of natural minerals which are not readily soluble in water, specifically, such minerals as natural polyhalite including potassium and magnesium in a ratio that is especially favorable in terms of assimilation by the plants. The present-day stocks of polyhalite ores available in the world for the production of chlorine-free fertilizers are ample. However, these stocks still remain dormant in view of the absence of any economically feasible technology.
Chlorine-free potassium fertilizers are known to be required for quite a variety of farming plants (vine, potatoes, citrus plants and others), on which chloride-ion has an inhibiting effect. The use of chlorine-free potassium fertilizers enables not only raising the yield of crops, but also improving the quality of farm products by virtue of an increase of starch content in potatoes, of sugar content in grapes and the like. These important considerations account for a steady world market demand for chlorine-free potassium fertilizers despite the fact that their cost as compared to that of potassium chloride is relatively high.
Moreover, the prior art methods of producing potassium sulphate, potassium nitrate or potassium phosphate based upon the use of potassium chloride involve corrosion of the equipment and a low degree of recovery of useful components.
The prior art also includes a number of other methods providing the production of potassium nitrate, e.g. by reacting potassium sulphate with magnesium nitrate, which so far have not gone beyond the scope of laboratory research.
The disadvantages inherent in the abovedescribed methods have been somewhat alleviated in another known method of producing a complex mineral fertilizer from polyhalite (see Pr. nauk Inst. technol. nieorgan. nawoz. mineral PWr 1973 No. 5, pp. 43-51). This method of producing a complex mineral fertilizer features the steps of decomposing polyhalite with the formation of a suspension, neutralizing said suspension and subsequently separating a resultant product. When practising the method, the step of decomposing polyhalite is conducted with the use of phosphoric acid at a temperature of 25.degree. C., while that of subsequently neutralizing the resultant suspension is effected with the aid of a 25% ammonia solution, whereafter the product is subjected to drying at a temperature of 70.degree. C. to produce a complex (nitrogen-phosphatic-potassic) fertilizer.
The NPK-fertilizer produced by this method has quite a few disadvantages manifesting themselves both in the fabrication of the fertilizer and in the application thereof.
In particular, the production of a ballast-free fertilizer is unattainable with this method inasmuch as calcium and sulphate-ions contained in polyhalite in amounts of up to 80% and qualified as ballast components are fully passed into the ultimate fertilizer, and the separation of said ballast components from the NPK-fertilizer during the course of polyhalite decomposition with phosphoric acid is technologically impracticable. Furthermore, the relatively low temperatures at which the decomposition of polyhalite is performed do not ensure an adequately high degree of recovery of useful components from the raw material processed.
The use of the NPK-fertilizer requires additional applications of potassium and nitrogen to the soil because the nutritives ratio thereof (up to 33.36% of P.sub.2 O.sub.5, up to 13.6% of K.sub.2 O, up to 4.2% of N) has an adverse effect on the growth of the plants.
All of the phosphorus in the NPK-fertilizer produced by this prior art method is in the form of water-insoluble calcium and magnesium salts, and all of the potassium and nitrogen is in the form of readily soluble potassium and ammonium sulphates. When applying such a fertilizer to the soil the soluble components of the fertilizer are being washed out from the soil due to the difference in solubility of the salts contained therein. This leads to further impairment of the NPK-fertilizer nutritives ratio, which, does not favor the growth of plants and causes much difficulty in use.